As is well known to those skilled in the art, cracked naphtha (obtained as product of a cracking operation or a coking operation) may contain a significant quantity of sulfur--up to as much as 13,000 wppm; and this material contributes a substantial quantity of undesired sulfur to the gasoline pool to which it is commonly passed. It is possible to decrease the sulfur content by (i) hydrotreating the whole feedstock to the cracking/coker unit or (ii) hydrotreating the product naphtha from these units.
The first noted alternative is a "brute force" effort that is very expensive in that it requires a large hydrotreater and it consumes significant quantities of hydrogen. The second-noted alternative is a more direct approach--but unfortunately it results in undesirable saturation of the olefins (typically originally present in amount of 20 w %-50 w %) down to levels as low as 4 w %-30 w %; and this reduces the octane number of the product gasoline by as much as 1-14 units. Prior art desulfurization of full range FCC naphtha from 13,000 wppm down to 20 wppm results in a typical decrease in octane number by about 14 units. This loss in octane number associated with desulfurization has a significant impact on the octane number of the refinery gasoline pool.
Typical prior art disclosures which are directed to hydrodesulfurization include
U.S. Pat. No. 4,140,626 Bertolacini and Sue-A-Quan) describe a selective hydrodesulfurization process employing a catalyst with a Group VI-B metal and a Group VIII metal deposited on a support consisting of at least 70 wt % magnesium oxide (MgO). Preferably, the Group VI-B metal is molybdenum and the Group VIII metal is cobalt. Catalyst A (a catalyst of the invention of this patent) is 3 wt % CoO/.about.16 wt % MoO3 on a pure MgO support. Catalyst B is a sample of commercial Criterion HDS-2A CoMo on alumina hydrotreating catalyst (with similar levels of CoO and MoO3). Catalyst A is better than Catalyst B for hydrodesulfurization (HDS). In addition, catalyst A produces better octane numbers than Catalyst B at equivalent values of HDS (in the range of 75-85% HDS); however, the improvement is only .about.1.5 octane numbers. Surprisingly, for both catalysts, olefin saturation is fairly low (&lt;.about.40 wt %) and octane penalties are fairly insignificant (&lt;.about.2 octane numbers) for the ranges of HDS studied. Other catalysts of the invention (prepared on supports with at least 70 wt % magnesium oxide) show HDS improvements.
U.S. Pat. No. 4,132,632 (Yu and Myers) is very similar to the above described patent except that the metals loadings are restricted to 4-6 wt % for the Group VIB metal and 0.5-2 wt % for the Group VIII metal. Again, preferably, the Group VI-B metal is molybdenum and the Group VIII metal is cobalt. Catalyst I (a catalyst of the invention of this patent) is .about.1 wt % CoO/.about.5 wt % MoO.sub.3 on a pure MgO support. Catalyst II is .about.3 wt % CoO/.about.17 wt % MoO.sub.3 on a support comprising 80 wt % MgO (i.e. a catalyst of Bertolacini, described above). Catalyst I generally gives poorer HDS than Catalyst II, but, Catalyst I gives less olefin saturation and better octane numbers at around the same level of HDS (.about.82-84%). The incremental octane improvement is small (.about.1.6 octane numbers). Again, for both catalysts, olefin saturation is fairly low (&lt;.about.40 wt %) and octane penalties are fairly insignificant(&lt;.about.2.6 octane numbers) for the ranges of HDS studied.
A paper entitled "DESULFURIZATION OF CAT CRACKED NAPHTHAS WITH MINIMUM OCTANE LOSS" was presented at the 1978 NPRA Annual Meeting in San Antonio, Texas by Coates, Myers and Sue-A-Quan. This paper presents a good overview of the development of what Amoco called their "Selective Ultrafining Process." The paper presented about one year before the above described patents issued, mentions two catalysts (presumably from the two patents). Sulfiding technique was mentioned as a major concern. Amoco claimed that the new catalysts showed lower rates of deactivation than standard hydrotreating catalysts for HDS. Incremental octane improvements were claimed to be 4 MON and 4.5 RON at 90% HDS. The incremental octane improvements of the presentation were much larger than those shown in the subsequent patents.
The conventional catalysts for naphtha hydro- treating include CoMo, NiMo, NiW, CoMoP, and NiMoP metal oxides supported on alumina such as the commercial Criterion C-444. Magnesia supported catalysts and silica-magnesia supported catalysts are disclosed in U.S. Pat. No. 2,853,429 and U.S. Pat. No. 3,269,938, respectively. The commercial BASF K8-11 catalyst, used in the water gas shift conversion, generally contains 4 wt % CoO and 10 wt % MoO.sub.3 on a magnesia-alumina-silica support. One of the common drawback of catalysts on magnesia-containing supports is the low HDS activity compared to alumina supported catalysts. It is believed that poor dispersion of MoO.sub.3 on magnesia supports and low surface area of magnesia supports contribute to the low HDS activity. British patent 2,225,731 discloses hydrotreating catalysts comprising Group VI-B and Group VIII metal hydrogenation components on a support which comprises magnesia and alumina in a homogeneous phase--which catalyst is said to have an activity comparable to similar catalysts based on alumina. U.S. Pat. No. 4,962,237 issued Oct. 9, 1990 to Dow Chemical Company as assignee of D. E. Laycock is of interest.
It is an object of this invention to provide an improved hydrodesulfurization process accompanied with minimum olefin saturation. Other objects will be apparent to those skilled in the art.